putting concrete in the right light!

Proper lighting is one of the core principles in concrete microscopy.

For the microscopic examination of concrete, a rectangular sample measuring 4x4x2 cm is typically cut and embedded in a special resin under vacuum (A). This has the dual benefit of fixing the sample in place and allowing the resin to penetrate the pore space of the concrete. As a result, the capillary porosity can be indirectly visualized (C). After hardening, the impregnated sample is glued to a slide and ground down to a thickness of 20 µm (B). The resulting thin section is now ready for extensive examination!

In this post, I want to focus on the sophisticated optics that enable fluorescence in thin sections. The resin used for making the thin section must contain special pigments that fluoresce when excited by a specific wavelength of light. For example, Struers' product shows an intense fluorescence in the wavelength range between 488 and 515 nm.

To make this emission usable in a standard polarizing microscope, filter cubes (blocking filters) with defined wavelength transmission are used in the reflected light path. It's important that the epoxy resin used in sample preparation is matched to the filter cube in the reflected light path. However, this only makes sense if a halogen light source is present, as LED lighting typically doesn't provide sufficient intensity in the desired wavelength range. Alternatively, a specialized UV light source can be used.

Depending on the configuration of the light source and filters, slight variations in fluorescence intensity may occur when examining the sample. This is essential when interpreting the capillary porosity in concrete, as the W/C ratio estimation depends on the intensity of the fluorescence. For this reason, I always recommend a trial phase for qualified interpretation of the W/C ratio and other concrete technological parameters when acquiring new equipment or training.

I hope I've sparked interest for more posts on the fascinating topic of concrete microscopy!